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United States Patent |
6,196,049
|
Schneider
|
March 6, 2001
|
Sensing element and method for manufacturing a sensing element
Abstract
A sensing element, in particular for an electrochemical sensor, for
determining the oxygen content of gases, includes at least one first
electrode exposed to a measured gas and at least one second electrode
exposed to a reference gas. A presintered support receiving a sensing
device is provided, a porous adhesion layer, also presintered, being
arranged between the support and the sensing device.
Inventors:
|
Schneider; Gerhard (Pettstadt, DE)
|
Assignee:
|
Robert Bosch GmbH (Stuttgart, DE)
|
Appl. No.:
|
195943 |
Filed:
|
November 19, 1998 |
Foreign Application Priority Data
| Nov 19, 1997[DE] | 197 51 128 |
Current U.S. Class: |
73/23.2; 204/425 |
Intern'l Class: |
G01N 019/10; G01N 027/26 |
Field of Search: |
73/23.2,31.05
204/425,412
|
References Cited
U.S. Patent Documents
4282080 | Aug., 1981 | Muller et al. | 204/412.
|
4300990 | Nov., 1981 | Maurer | 204/412.
|
4334974 | Jun., 1982 | Muller et al. | 204/425.
|
4505807 | Mar., 1985 | Yamada | 204/425.
|
4610741 | Sep., 1986 | Mase et al. | 156/89.
|
Primary Examiner: Williams; Hezron
Assistant Examiner: Politzer; Jay
Attorney, Agent or Firm: Kenyon & Kenyon
Claims
What is claimed is:
1. A sensing element, for an electrochemical sensor, for determining an
oxygen content of a gas, comprising:
at least one first electrode exposed to a measured gas;
at least one second electrode exposed to a reference gas;
a sensing device;
a sintered support receiving the sensing device when the sensing device is
in an unsintered state; and
a sintered, porous adhesion layer situated between the support and the
sensing device.
2. The sensing element according to claim 1, further comprising a heating
device, the heating device and the sensing device being on opposite sides
of the support.
3. The sensing element according to claim 1, wherein:
the at least one first electrode and the at least one second electrode
correspond to individual functional layers arranged one above the another
according to a preselected layout,
the functional layers are at least one of successively printed and
successively pressed onto the sintered support, and
the functional layers are sintered with the support.
4. The sensing element according to claim 3 wherein:
the adhesion layer that is sintered with the support is arranged onto the
support.
5. The sensing element according to claim 4, wherein:
a base layer of the sensing device is pressed into the adhesion layer.
6. The sensing element according to claim 3, further comprising:
a heating device applied onto the support independently of the functional
layers.
7. The sensing element according to claim 6, wherein:
the heating device is applied on a side of the support facing away from the
functional layers.
8. The sensing element according to claim 3, wherein:
the heating device is sintered with the support.
Description
FIELD OF THE INVENTION
The present invention relates to a sensing element, in particular for an
electrochemical sensor, for determining the oxygen content of gases, as
well as to a method for manufacturing the sensing element.
BACKGROUND INFORMATION
Sensing elements are known. They are configured, for example, as so-called
planar sensing elements, which have a first electrode exposed to a
measured gas, on a solid electrolyte configured as a support, and a second
electrode exposed to a reference gas. In a number of applications, the
sensing element must be heated to a specific temperature. It is known for
this purpose to associate with the sensing element a heating device, which
usually has heating conductors running below the electrode that is exposed
to the reference gas.
In order to deliver a reference gas onto the reference gas electrode, a
reference gas conduit which extends in the longitudinal direction of the
sensing element is provided inside the layered, planar sensing element.
To manufacture sensing elements of this kind, it is known that the
individual functional layers yielding the sensing element are arranged one
above another as so-called green films, the individual functional layers
having a specific layout corresponding to the structure of the sensing
element. The entire sensing element is then sintered. It is
disadvantageous in this context that, because the functional layers are
present as green films, the sensing element is relatively labile; handling
both during application of the functional layers and during sintering can
thus be performed only with the greatest of care in order to prevent
damage to the sensing element.
SUMMARY OF THE INVENTION
The sensing element according to the present invention offers, in contrast,
the advantage that the manufacture and handling thereof are simplified.
Because the sensing element is patterned on a presintered support, a
porous adhesion layer being arranged between the support and the sensing
element, a relatively solid support, which is easy to handle and at the
same time protects the applied functional layers of the sensing element
from mechanical damage, is available both while the individual functional
layers of the sensing element are being printed on, and during subsequent
sintering of the functional layers.
In a preferred embodiment of the present invention, provision is made for
the support to have on its one side a sensing device and on its other side
a heating device. It thereby becomes advantageously possible to decouple
the manufacture of the heating device from the manufacture of the sensing
device, so that they can be accomplished in separate process sequences. In
addition to the resulting optimization of both the application of the
heating device and the application of the sensing device onto the opposite
sides of the support, an increase in yield can also be attained, since
when the heating device and the sensing device are manufactured in
succession, any heating devices that may be manufactured defectively no
longer need to be equipped with the sensing device. This allows not only
material but also time and cost to be saved when manufacturing the sensing
elements.
In addition, the method according to the present invention for
manufacturing the sensing element offers the advantage that the time for
sintering the sensing element can be reduced. Because the support
substrate is already pre-sintered, all that is necessary is a
post-sintering of the applied functional layers. Since the latter are
relatively thin, the sintering time can be kept correspondingly short.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a sectioned depiction through a sensing element in a first
exemplary embodiment of the present invention.
FIG. 2 shows schematic views of the individual functional layers of the
sensing element according to FIG. 1.
FIG. 3 shows a sectioned depiction through a sensing element in a second
exemplary embodiment of the present invention.
FIG. 4 shows schematic depictions of the individual functional layers of
the sensing element according to FIG. 3.
DETAILED DESCRIPTION
FIG. 1 shows a sectioned depiction of a sensing element 10. Sensing element
10 includes a support 12 which has on its one side 14--in this case on
top--a sensing device 16, and on its other side 18 a heating device 20.
Sensing element 10 possesses a planar layered structure which extends from
a measured-gas-side section shown in FIG. 1 to a reference-gas-side
section (not depicted) remote from the measured gas. Sensing element 10 is
a constituent of an electrochemical sensor (not depicted), and is secured
(sealed) in a housing of the sensor. The sensor is exposed to a gas to be
measured, for example to the exhaust gas of a motor vehicle. The structure
and function of electrochemical sensors which use a Nernst element to
analyze a partial pressure difference between a reference gas and a gas to
be measured, and make available a corresponding signal, are known.
Sensing device 16 possesses a first electrode 22 which can be exposed to
the measured gas, for example to the exhaust gas of a motor vehicle.
Electrode 22 is arranged on a solid electrolyte 24 on whose side facing
away from electrode 22 a second electrode 26 is arranged. Electrode 26 can
be exposed to a reference gas, for example to atmospheric oxygen. A
reference gas conduit 28, which extends in the longitudinal direction of
sensing element 10 and terminates at the section of sensing element 10
remote from the measured gas, is configured in order to deliver the
reference gas. Electrode 26 is of split configuration on the
measured-gas-side section, so that it extends out into two arms 30. Arms
30 are partially covered by ion conductors 32 which are arranged in
columnar fashion between electrode 26 and the solid electrolyte. Electrode
22 is surrounded by a protective layer 34 which has a minimum porosity
allowing a measured gas to come into contact with electrode 22. A partial
pressure difference that is established between electrodes 22 and 26
results, via ion conductors 32 and solid electrolyte 24, in an exchange of
charge carriers which results in the pickoff of a signal at electrodes 22
and 26, at their connecting contacts (not depicted in FIG. 1) remote from
the measured gas.
A porous adhesion layer 36 and a gas-tight base layer 38 are arranged
between carrier 12 and sensing device 16.
Heating device 20 has a heating conductor 40, arranged for example in
meander fashion, which is covered over by an impervious heater cover layer
42.
According to one exemplary embodiment, support 12 includes an aluminum
oxide (Al.sub.2 O.sub.3)/ZrO.sub.2 substrate, adhesion layer 36 of a
porous aluminum oxide (Al.sub.2 O.sub.3) layer, base layer 38 of an
yttrium-stabilized zirconium layer (ZrO.sub.2 /Y.sub.2 O.sub.3), solid
electrolyte 24 of stabilized zirconium oxide, and ion conductors 32 and
protective layer 34 of porous zirconium oxide. Electrodes 22 and 26
include, for example, platinum-cermet conductor paths.
Heating conductor 40 also is composed, for example, of a platinum conductor
path, while cover layer 42 is composed of an impervious aluminum oxide
(Al.sub.2 O.sub.3).
The manufacture of sensing element 10 shown in FIG. 1 will now be discussed
with reference to FIG. 2.
At the outset, support 12 is available as an already sintered aluminum
oxide substrate. Support 12 is equipped at least on its side 14,
optionally also on side 18, with an adhesion layer 36 that is also already
pre-sintered. Support 12 thus forms a relatively stable substrate for the
subsequent patterning of sensing device 16.
Provision is preferably made, if sensing element 10 is to have a heating
device 20, for heating device 20 to have been applied prior to the
patterning of sensing device 16. For this, heating conductor 40 and cover
layer 42 are printed onto side 18 of support 12 in successive printing
steps, and co-fired together with side 18. The result of this is that the
manufacture of heating device 20 is completely decoupled from the
manufacture of sensing device 16. The process steps for the manufacture of
heating device 20 can thus be performed independently of any process steps
for the manufacture of sensing device 16 that may occur later, and can be
optimized without consideration of those process steps.
There are thus two ways of arriving at a pre-sintered support 12. According
to the first variant, a film yielding support 12 is equipped with the
porous adhesion layer 36, and the film is sintered together with adhesion
layer 36 at approximately 1600.degree. C. The second possibility is to
equip the film yielding support 12 with the porous adhesion layer 36 and
with heating conductors 40 and cover layer 42, and to sinter this
composite at approximately 1600.degree. C., thus making available for the
further preparation of sensing device 16 a pre-sintered support 12 having
an already patterned and co-sintered heating device 20.
As FIG. 1 elucidates, the manufacture of sensing elements 10 can take place
in a so-called multiple panel; i.e., in parallel process steps, a
plurality of sensor elements 10 are produced simultaneously as a result of
the successive patterning of the individual functional layers, the
individual sensing elements 10 being achieved by subsequent isolation.
FIG. 1 indicates that a plurality of sensing elements 10 are
simultaneously patterned, next to one another or in front of and/or behind
one another, in a specific grid spacing as viewed from above. Isolation
can be accomplished, for example, by breaking the substrate of support 12
at indicated break edges 44, which preferably are produced as support 12
is being patterned.
Because the production of heating device 20 is decoupled in
process-engineering terms from that of sensing device 16, heating device
20 can very advantageously first be tested, so that, for example on
supports 12 having a defective heating device 20, patterning of a sensing
device 16 on side 14 opposite heating device 20 can be omitted. This makes
it possible to achieve an increase in the yield of the materials used to
produce the individual functional layers, since a sensing device 16 is no
longer applied onto sensing elements 10 that have already been recognized
as defective. In the case of manufacture in a multiple panel,
corresponding recognition and microprocessor-controlled patterning of
sensing elements 10 can be used to remove support 12 having the defective
heating device 20 from the process of patterning sensing device 16.
According to further exemplary embodiments, of course, it is possible
first to pattern sensing device 16, and then to pattern heating device 20
on the opposite side 18. Here again, analogously, patterning of a heating
device 20 on a support having a defective sensing device 16 can be
dispensed with. Increased yields of the materials used are obtained in
this case as well. For the case in which support 12 is first equipped with
sensing device 16, a film of highly sinterable aluminum oxide (Al.sub.2
O.sub.3), which for example sinters in impervious fashion at a sintering
temperature of approximately 1400.degree. C., can be used as the starting
material for support 12. For this purpose, this highly sinterable aluminum
oxide is equipped with the individual layers yielding sensing device 16,
then sintered at approximately 1400.degree. C., and subsequently heating
device 20 is once again produced.
In the patterning of sensing device 16, base layer 38 is first printed, for
example by screen printing, onto the pre-sintered composite of support 12
with the porous adhesion layer 36 and optionally with heating device 20,
and is then pressed into the pre-sintered porous adhesion layer 36. This
results in an intimate bond between support 12 and sensing device 16 which
persists even during later use of sensing element 10 as intended. In
successive printing steps, second electrode 26 is then first printed on,
forming its arms 30, followed by a sacrificial layer 46 yielding reference
gas conduit 28. Then ion conductors 32, solid electrolyte 24, first
electrode 22, and protective layer 34 are printed on. Protective layer 34
is printed on in sub-steps, so that on the one hand the actual electrode
22, and also a conductive path 48 which connects to a connecting contact
50, remote from the measured gas, of electrode 22, are covered.
The schematic plan view of the individual layers shown in FIG. 2 depicts
the measured-gas-side section of a sensing element 10 on the left, and its
section remote from the measured gas on the right. The layout of the
individual functional layers is such that the structure shown in section
in FIG. 1 is created in the measured-gas-side section of sensing element
10, while in the section remote from the measured gas, connecting contacts
50 of electrode 22 and 52 of electrode 26 are exposed for making contact
with an analysis circuit (not depicted). The thicknesses of the individual
functional layers, in particular of solid electrolyte 24 and protective
layer 34, are designed to be such that lateral envelopment of electrodes
22 and 26 occurs, i.e. that their outer end surfaces extending in the
longitudinal direction of sensor element 10 are covered over.
After application of the functional layers of sensing device 16 onto
support 12, the entire sensing element 10 is sintered, support 12 and
adhesion layer 36, as well as optionally heating device 20, already being
pre-sintered. Sintering is accomplished at a temperature of, for example,
1300 to 1500.degree. C. Once sintering has occurred, sensing elements 10
are isolated from the overall multiple panel by isolating supports 12 at
break edges 44 by applying a small force. During sintering, sacrificial
layer 46 yielding reference gas conduit 28 is completely dissolved away.
This layer can be composed, for example, of carbon, carbon black,
theobromine, or other suitable materials.
Because sensing devices 16 are patterned onto an already pre-sintered
support 12, handling of the entire multiple panel is on the one hand
simplified, since the inherently relatively stable support 12 is available
for holding and/or transportation. In addition, sintering can be
accomplished in a relatively short time period, since support 12 is
already sintered, and a correspondingly shorter time suffices for complete
sintering of the functional layers of sensing device 16. As compared with
known manufacturing methods, only proven and easily controllable process
steps, such as printing, pressing, and sintering, are necessary. Any
punching operations, through-plating, or cutting operations in order to
isolate sensing elements 10, which are relatively incompatible with the
manufacturing methods used, can be dispensed with.
FIG. 3 shows a further sectioned depiction through a sensing element 10,
which although it has a modified structure is equipped with the same
reference characters as in FIG. 1, which will not be explained again. Only
those differences which exist will therefore be discussed. In contrast to
the cross section shown in FIG. 1, FIG. 3 shows a longitudinal section
through a sensing element 10. Electrodes 22 and 26 are configured here as
comb electrodes lying in one plane, i.e. fingers of electrodes 22 and 26
extending from a base are alternatingly arranged next to one another, in
staggered fashion, in the longitudinal extension of sensing element 10.
This makes it possible to apply electrodes 22 and 26 in a single printing
step. FIG. 4 shows, by analogy with FIG. 2, the individual printing steps
to produce sensing device 16. Base layer 38 is first printed onto support
12 (not depicted here) with its porous adhesion layer 36, and pressed into
adhesion layer 36. Electrodes 22 and 26, sacrificial layer 46, solid
electrolyte 24, and protective layer 34 are then printed on. All the other
process steps are analogous to the exemplary embodiment explained with
reference to FIGS. 1 and 2.
Ion conduction between electrodes 22 and 26, as defined by the exemplary
embodiment in FIG. 3, takes place via base layer 38, so that a signal can
be picked off at connecting contacts 50 and 52. Solid electrolyte 24
arranged above the fingers of electrode 26 simultaneously constitutes
reference gas conduit 20 and a cover for reference gas conduit 20 and
electrode 26 with respect to an external measured gas.
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